Signaling method and apparatus



Oct. 6, 1936. LE RoY A. MaccoLL 2,056,284

SIGNALING METHOD AND APPARTUS Filed June 7, 1955 lg@ y 5H lm All:

VAVAVA'A/ /NvE/vro L. A. MACCLL ATTORNEY Patented Oct. 6, 1936 `UNITEDsTATEs PATENT OFFICE SIGNALING METHOD AND APPARATUS Application June 7,1935, Serial No. 25,430

6 Claims.

The present invention relates tc signaling through media which distortthe signal waves and to method and means for compensating or correctingfor the distortion to a substantial extent.

Distortion in a transmission medium is either linear or non-linear. Inthe former no new frequencies are produced by the transmission medium.In the latter new frequencies are produced. In the former, waves ofseveral frequencies simultaneously present, add algebraically to givethe resultant arrival wave. In the latter there is also a multiplicationof wave components, and the new frequency components referred to are theproducts of such multiplication.

The present invention deals only with linear distortion. In linearsystems of some types and ranges of constants, the phase and amplitudedistortion may be excessive, resulting in arrival waves of forms thatare different from that of the sent impulses during the periodcorresponding to the duration of the sent impulse and that also persistbeyond the time corresponding to the duration of the sent impulse. If,then, two impulses are sent out with an interval of no impressed currentbetween them, the arrival current or voltage wave may, for example, notfall to zero between the wave portions corresponding to the two sentimpulses, but the receiver may have applied to it a voltage of somevalue more or less all the time, rendering reception of discreteimpulses diicult. The arrival wave resulting from a single sent impulsemay even extend over into or beyond the time interval correspending tothe next sent impulse, and for a sent impulse of given polarity thearrival wave may undergo reversals of polarity during times later thanthe time corresponding to the duration of the sent impulse. An impulseapplied to the system may give rise to transient eiects which have theappearance of a whole succession of impulses.

The portion of the arrival wave which persists beyond the period oflduration corresponding to that of the sent impulse is termedintersymbol interference. It is the part of the wave which tends to fillup the interval between the sent impulses when the current or voltageshould fall to zero.

An object of the present invention is to reduce intersymbolinterference.

The manner of carrying out this and other objects of the invention, aswell as an understanding of the invention and its various aspects andobjects, will be had from the following detailed description taken inconnection with the accompanying drawing, in which Figs. 1 and 2 showdiagrams of wave forms to be used in the description,`and

Fig. 3 is a simplified schematic circuit diagram (Cl. P18-22) of onesystem embodying the invention by way of example.

It is theoretically possible according to the invention totransmit'signals over any physical transmission system at any speed andin such manner that the message at the receiving end has the sameduration as the message at the sending end. The theoretical possibilityis based on the assumed validity of ordinary circuit theory.Practically, an attempt to obtain exceedingly high speed would run intodiculties due to the electronic effects.

It is to be understood throughout the discussion that only real physicalsystems are under consideration; no ideal apparatus, such as filterswhich absolutely suppress certain bands of frequencies, are postulated.The transmission system proper, that is aside from the sending andreceiving apparatus, is assumed to be linear.

To simplify the exposition, consider first direct current telegraphywith machine sending so that accurately timed signals of equal impulseduration are sent. At the sending end of the system any messageexpressed, say, as a potentialtime curve, is constant in each of thetime intervals 0 t r, 1 t 2r, 2f t 3r, but the constant value may bedifferent in different intervals. From the physical point of View thereis no difference between one message and several successive messages;hence there is no loss in generality in confining the discussion to asingle message.

The quantity f, the duration of a signal element at the sending end, isquite arbitrary, but in the case of any one particular system it is setonce for all. It is understood that the constant value which the curvehas in any one interval is limited to one of a finite number of possiblevalues, including the value zero. There is no loss in generality inassuming that the message has the value zero for all negative values oft. A simple example of such a message is shown in Fig. l.

Now consider the single element of the sent message Which occupies theinterval 7`1 t (9'+1)r, where 7' is zero or an arbitrary positiveinteger. This element is shown at A in Fig. 2, and it will be denoted byflut-if). This element will give rise to a particular elementary currentwave at the receiving end which will be denoted by f(t-jr). (The delayin the transmission system has been neglected here. This amounts tousing different zeros of time at the two ends of the system.) Theimportant properties of the latter function are that it vanishes for t1r, that it has appreciable values for some values of t greater than(j-l-Dr, and that its form diiers more or less Widely from that offut-ir). A typical example of ,Kt-if) might have a form somewhat likethat shown at B in Fig. 2. Each element of the message at the sendingend will give rise to such an elementary current at the receiving v moreor less diicult.

` the iirst element.

Consider the function fut-if) which is obtained from Ht-ir) by makingthe function equal to zero in the interval 7'1 t (7 +1) v. In the casein which ,Kt-ir) is as shown at B in Fig. 2, the function f1(t-7`r) isas shown at C in Fig. 2. It is easy to imagine a machine whichwouldgenerate a current 'wave having the form fut-if). For example. thiscould be done by a properly shaped template driving a contact over arheostat. Such a machine will be called a corrector. Assume that at thereceiving end there are as many correctors, say N, as there areintervals of length f in which fut-ir) is appreciably different fromzero. Assume that the first corrector is set to generate ithe currentsCn,`(t), Cima-N1) Cisf1(t-2Nr), that the second corrector is set togenerate the currents C'zifdt-f), Czzfi (t-r-Nf) C'zaf1(t-r-2N-r), thatthe third corrector is set to generate the currents Csifi (t-Z-r)CazfiQ-ZT-Nr) Casf1(t-2r-2Nr) and so on, Where the Cs are, for thepresent. arbitrary constants.

- Now consider the reception of the message. There is no intersymbolinterference during the interval 0 t r; the current during that intervaldepends only on the magnitude of the first element in the sent message.Consequently. any particular measurement on the current during thatinterval, say of its integral over the interval or of its magnitude atthe center or latter end of the interval, gives complete knowledge ofthe first element of the sent message. Let this measurement set theiirst corrector to generate the current C11fi(t) where C11 is given avalue proportional to the magnitude of the first element. This settingis maintained until t=(N+1) -r when the corrector is reset in a mannerto be described presently. The current generated by the corrector can besubtracted from 'the received signal wave, with the result that allsignal elements following the first are freed from the intersymbolinterference due to the first. 'Ihe second element in the receivedsignal is now free from* intersymbol interference, and lany particularmeasure- 'ment on it will determine the magnitude of the the magnitudeof the second element. This setf ting is maintained until t (N4-2) r.When the current generated by the corrector is subtracted from thereceived signal the signal is freed from intersymbol interference due tothe second element. 'I'his process is continued until all of thecorrectors are in operation.

After t=Nr there is no need to continue the corection for theintersymbol interference 'due' to Hence, the measure of the (N+1)thelement of the signal is caused to 'set the rst corrector to generatethe current Cizfi(t-Nr), where C12 is proportional to the magnitude ofthe (N+-Du* element, and this current ls subtracted from the signal waveto eliminate the intersymbol interference due to that element. Likewise,the determination of the magnitude of the (N+2) mi element is caused toset the second corrector to eliminate the intersymbol interference dueto that element. And so' the process continues.

In this way the message can be received without intersymbol interferenceand interpreted. It is to be noticed particularly that the transmissioncharacteristics of the transmission system have no effect on the speedof signaling, and that the only Way in which they enter is in theircontrolling influence on the number and construction of the correctors,within the limitations noted above in case of exceedingly high signalspeeds. Furthermore, it is to be noted that the duration of thecorrected received signal is precisely equal to that of the sent signal.Hence we have the proposition: it is possible, using only apparatuswhich is in principle easily constructed, to signal through anytransmission system at any speed and in such a manner that the timerequired to receive and interpret the message is equal to the timerequired to put the message on the line.

The foregoing general description of the principle of the inventionhaving been given, a practical embodiment of the invention in the formof a telegraph vsystem will now be described as illustrated in Fig. 3.

'I'his figure shows a transmitting station at the left and a receivingstation at the right, connected by the line I0. which may be a land lineor a submarine cable. Oppositely poled batteries II and I2 have a commonground and their ungrounded poles are connected to the brushes of arotating commutator I3 carrying on its same shaft a drive for a sendingtape I4 which is assumed to be punched to represent signals. Brushes I5reach through the punched holes and make contact with metal platen I6causing a positive or a negative impulse to be sent to line depending onthe position of commutator I3 when a hole in the tape is underneath abrush I5. This showing of the sender is merely symbolic and is intendedto represent any suitable type of which many are known in the art. Withthe type of sender indicated, a space is denoted by zero current, a dotby one polarity of current and a dash by the opposite polarity. Theduration of all the symbols is the same.

A positive impulse as sent is of the form shown at A in Fig. 2. Asreceived, the impulse is assumed to have the form shown at B. Thisrepresents intersymbol interference extending throughout two signalintervals after the interval corresponding to the sent signal. Hencethree correctors of the type above discussed are provided at thereceiving station.

These are shown as 'comprising rotating cam ,mechanisms and associatedelements, the cams being shown at 20, 2l, and 22. As these cams rotatethey cause contacts `24, 25, and 26, respectively,`to move alongpotentiometer resistances 28, 29, and 30 for impressing correctingimpulses on the amplifiers 32, 33 and 34. The potentiometers derivetheir voltages from condensers 36, 3l, and 38 in a manner to bedescribed.

It is thought that the construction can best be understood by tracingthrough the operative steps. For simplicity it will be assumed that thefirst impulse is ybeing received over the system and that, therefore,there is no intersymbol in- 1 fixed contacts connected across resistance43 Y placing a charge on condenser 36 of a voltage corresponding to thatof the received impulse. Immediately thereafter in theircounter-clockwise movement switches I, 52 connect condenser 36 acrossresistance 28, which is so high as not to cause appreciable discharge ofthe condenser during the correcting period. At this time, that is,during the signal arrival interval designated interval #I in Fig. 2, cammechanism 20 is oper- "ating over a circular Vportion-of its surface andcontact 24k is held oi the end of resistance 28 so that no voltage isimpressed on amplifier 32. Referring to curve C, Fig. 2, zero correctingvoltage is being developed during interval #L At the end of thisinterval, however, a highregion on cam 28 begins to move contact 24along `resistance 28, applying initially a relatively high voltage toamplifier 32, the amplified output of which is impressed on theresistance 42 in such phase as just to nullify the intersymbol voltagerepresented at B, Fig. 2. As cam 28 continues to move contact 24 alongresistance 28 a voltage is developed in the input to amplifier 32 whichfollows in magnitude and sign the form of the correcting curve C, Fig.2. Reversal of sign occurs when contact 24 passes the center ofresistance 28 at the end of interval #2. Toward the end of interval #3,switches 5I, 52 place a short circuit across condenser 36 and dischargeit in preparation to receive a new charge when these switches connect itagain to line resistance 43.

If only the one impulse had been sent as assumed in the three intervalsthat have been considered, condensers 31 and 38 would not receive anycharge from the line and cam mechanisms 2| and 22 would be of no effect.However, with successive impulses on the line these mechanisms operatein succession in the same manner as has' been described of cam 20 andits associated elements. The sequence of events is then as follows: Thefirst signal element is free from intersymbol interference. Cam 20 asdescribed generates correcting potentials for intervals #2 and #3.'I'his insures that interval #2 is free of intersymbol interference. Ifinterval #2 contains a received signal element, cam 2| and associatedmechanism generates correcting currents for intervals #3 and #4, leavinginterval #3 free. If interval #3 contains a received impulse,

cam mechanism 22, etc., 'corrects intervals #4 and #5, leaving interval#4 free of interference. Cam mechanism 20, etc., then can operate asbefore to correct intervals #5 and #6, leaving interval #5 free, and soon.

While the invention has been illustrated and described as applied to atelegraph system, it is not to be construed as limited to any particulartype or form of signaling system, since it is capable of generalapplication as demonstrated in the discussion heretofore given. Itsapplication in practice to systems employing complicated wave forms islimited only by the dimculty oi providing correctors of sumcient numberand complexity to effect the necessary correcting operations.

One important aspect of the invention is the secrecy of transmissionwhich it makes possible. The transmission band of the line may be madeso narrow in comparison with the speed of signaling that the intersymbolinterference will be sufciently great to make the signals incapable ofintelligible reception without the aid of specially constructedcorrectors in the general manner herein disclosed.

What is claimed is:

1. The method of signaling over a medium which produces lineardistortion in the transmitted waves comprising determining thecharacteristic distortion of an arrival wave in the intersymbolintervals only, producing at a receiving point a compensatory typeofwave, and .Y

opposing the compensatory wave to the arrival wave to reduce intersymbolinterference.

2. In a signaling system, means at a sending station to impress impulsesof current on a line in a succession of signal time intervals fortransmission thereover, said line producing linear distortion in thetransmitted impulses such that the arrival wave resulting from a sentimpulse of one polarity is prolonged beyond the time intervalcorresponding to the sent impulse and over into the time intervalcorresponding to the next sent impulse and undergoes a reveral ofpolarity, and means at a receiving station on said line forsubstantially annulling the prolonged effect of such an arrival wavebeyond the time interval corresponding tothe sent impulse.

3. The method of secret signaling over a. system producing lineardistortion but negligibly small non-linear distortion comprisingtransmitting at a signaling speed so high that the intersymbolinterference eiectively masks the signals and at a distant receivingpoint of said system reducing the intersymbol interference sumciently toenable the signals to be interpreted.

4. In a. system for transmitting signal impulses with linear intersymboldistortion, means at a receiving point for producing a voltage wavecorresponding in magnitude and polarity to the intersymbol interferenceproduced by each elemental signal impulse to be received and means forcombining received waves with'said produced waves in such phase as tocompensate the intersymbol interference while preserving the signalimpulses.

5. In a signaling system producing iinear distortion in transmittedwaves, means at a receiving point for producing waves of magnitude andform predetermined from the distortion characteristic of the system,means determining the operation of said rst means in the intervalcorresponding to that in which a signal impulse is being sent, and meanscausing said first means to begin to operate to neutralize receivedwaves at the end of said interval.

6. In a signaling system producing linear distortion in transmittedwaves, means at a receiving point timed in its operation to receive avoltage from the line during the interval corresponding to the intervalof a sent impulse and to produce therefrom a wave similar in shape tothe distortion, and means opposing such produced wave to the receivedwave to compensate such distortion.

Ls ROY MAcCOLL.

